1. Field of the Invention
The invention is directed to compositions and methods for improving the physicomechanical properties and contrast ratio of displays and also to semi-finished and finished display panels having improved physicomechanical properties and their manufacture.
2. Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. An electrophoretic fluid composed of a colored solvent with charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side or the other causing either the color of the pigment particles or the color of the solvent being seen from the viewing side.
There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8): 1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of particles, such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
An improved EPD technology is disclosed in U.S. Pat. Nos. 6,930,818, 6,672,921, 6,933,098, 6,545,797 and 7,005,468, and US Publication Nos. U.S. 2004-0085619 and 2004-0112525, all of which are incorporated herein by reference.
A typical microcup-based display cell, is shown in FIG. 1. The cell (10) is partitioned by walls (10b) into subcells or microcups (10a) and sandwiched between a first electrode layer (11) and a second electrode layer (12), at least one of which is transparent. A primer layer (13) is optionally present between the cell (10) and the first electrode layer (11). The subcells or microcups (10a) are filled. with an electrophoretic fluid comprising pigment particles (10c) dispersed in a dielectric solvent (10d). The filled microcups are sealed with a sealing layer (14) and laminated with the second electrode layer (12), optionally with an adhesive (15). In the case of in-plane switching EPDs, both in-plane electrodes may be on the same side of the EPD and one of the electrode layers mentioned above may be replaced by an insulating substrate.
The display panel may be prepared by microembossing or photolithography as disclosed in U.S. Pat. No. 6,930,818. In the microembossing process, an embossable composition is coated onto the conductor side of the first electrode layer (11) and embossed under pressure and/or heat to produce an array of microcups.
The embossable composition may comprise a thermoplastics, thermoset or a precursor thereof which may be selected from the group consisting of multifunctional acrylates or methacrylates, vinylbezenes, vinylethers, epoxides, oligomers or polymers thereof, and the like. Multifunctional acrylates and oligomers thereof are the most preferred. A combination of a multifunctional epoxide and a multifunctional acrylate is also very useful to achieve desirable physico-mechanical properties. A crosslinkable oligomer imparting flexibility, such as an urethane acrylate or polyester acrylate, is usually also added to improve the flexure resistance of the microcups. The composition may contain an oligomer, a monomer, additives and optionally a polymer. The glass transition temperature (Tg) for the embossable composition usually ranges from about −70° C. to about 150° C., preferably from about −45° C. to about 50° C.
The microembossing process is typically carried out at a temperature higher than the Tg of the embossable composition. A heated male mold or a heated housing substrate against which the mold presses may be used to control the microembossing temperature and pressure.
The mold is released during or after the embossable composition is hardened to reveal the subcells or microcups (10a). The hardening of the embossable composition may be accomplished by cooling, solvent evaporation, cross-linking by radiation, heat or moisture. If the curing of the embossable composition is accomplished by UV radiation, UV may radiate onto the thermoplastic, thermoset or precursor layer through the transparent conductor layer. Alternatively, UV lamps may be placed inside the mold. In this case, the mold must be transparent to allow the UV light to radiate through the pre-patterned male mold on to the embossable composition.
A thin primer layer (13) is optionally precoated onto the electrode layer (11) to improve the release properties of the mold and the adhesion between the subcells or microcups (10a) and the electrode layer (11). The composition of the primer layer may be the same or different from the embossing composition.
In general, the dimension of each individual microcups or subcells may be in the range of about 102 to about 106 μm2, preferably from about 103 to about 5×104 μm2. The depth of the cells is in the range of about 3 to about 100 microns, preferably from about 10 to about 50 microns. The ratio between the area of opening to the total area is in the range of from about 0.05 to about 0.95, preferably from about 0.4 to about 0.9. The width of the openings usually are in the range of from about 15 to about 500 microns, preferably from about 25 to about 300 microns, from edge to edge of the openings.
The microcups are filled with an electrophoretic fluid and top-sealed by one of the methods as disclosed in U.S. Pat. Nos. 6,930,818 and 7,005,468, the contents of which are incorporated herein by reference. For example, it may be accomplished by a two-pass method involving overcoating the filled microcups with a top-sealing composition comprising a solvent and a top-sealing material. The top-sealing composition is essentially incompatible with the electrophoretic fluid and has a specific gravity no greater than that of the electrophoretic fluid. Upon solvent evaporation, the sealing composition forms a conforming seamless seal on top of the electrophoretic fluid. The top-sealing layer may be further hardened by heat, radiation, e-beam or other curing methods. Sealing with a composition comprising a thermoplastic elastomer is particularly preferred. Alternatively, the top-sealing may be accomplished by a one-pass method in which the sealing composition is dispersed in an electrophoretic fluid and together with the electrophoretic fluid is filled into the microcups. The top-sealing composition is essentially incompatible with the electrophoretic fluid and is lighter than the electrophoretic fluid. Upon phase separation and solvent evaporation, the top-sealing composition floats to the top of the electrophoretic fluid and forms a seamless sealing layer thereon. The top-sealing layer may be further hardened by heat, radiation or other curing methods.
The top-sealed microcups finally are laminated with the second electrode layer (12) optionally pre-coated with an adhesive layer (15).
Transmissive or reflective liquid crystal displays may also be prepared by the microcup technology as disclosed in U.S. Pat. No. 6,795,138 and U.S. Publication. No. 2004-0170776, the contents of which are incorporated herein by reference.
The displays prepared from the microcup and top-sealing technologies represent a significant advancement in the field of display technology. The microcup-based display may have an adhesive layer and a sealing layer and most of the commonly used adhesives may exhibit a strong capacitor effect. The use of a hydrophilic adhesive or addition of a conductive additive in the adhesive may alleviate the problems associated with the capacitor effect, but these possible remedies often result in setbacks such as sensitivity to humidity, undesirable current leakage or short circuitry.
In US Publication No. 2004-0112525, the content of which is incorporated herein by reference, a method for improving the adhesion properties and switching performance of electrophoretic displays is disclosed. The method involves utilizing a composition comprising a high dielectric polymer or oligomer and optionally a crosslinking agent as an adhesive or top-sealing layer. In the method disclosed, a thermal hardening step is typically required. Unfortunately, thermal hardening is a very slow process particularly at a low temperature typically employed to avoid undesirable evaporation of the dielectric solvent in the electrophoretic fluid. A catalyst for the crosslinking reaction may be used to speed up thermal curing, however, at the expense of the green time of the coating solution. The low thermal curing temperature also results in a low Tg of the cured top-sealing or adhesive layer because of the vitrification effect—the thermal curing reaction will slow down significantly when Tg of the curing system is approaching the curing temperature. A low Tg top-sealing or adhesive layer therefore results in deteriorated EPD temperature latitude probably because the pigment particles tend to irreversibly stick to the top-sealing layer when the operation temperature is approaching the Tg of the top-sealing material.
The other disadvantage of the thermally cured top-sealing/adhesive layer is the short green time for the subsequent lamination onto the electrode layer or supporting substrate. As a result, the display panels manufactured with the thermally cured sealing or adhesive layer are often finished display panels with the electrode layer (12) laminated before being shipped to customers. This finished or prelaminated structure requires different electrode patterns or designs predetermined at the time of panel manufacturing to meet different customer specifications. For electrophoretic or liquid crystal displays that require a common, non-patterned electrode layer or an insulating substrate on one side, it is highly desirable to streamline the manufacturing operation by supplying to customers a semi-finished display panel in a jumbo roll which comprises filled and sealed microcups laminated with a temporary substrate such as a release liner to prevent the sealing or adhesive layer from sticking to the back of the roll. Upon receiving the roll of the semi-finished display panel, customers may cut it into the desired format and size, remove the temporary substrate to expose the sealing or adhesive layer, and laminate the panel onto a second electrode layer with a desired electrode design to complete the display panel assembling for various applications. Alternatively, the second substrate or electrode layer may be disposed onto the sealed microcups by a method such as coating, printing, vapor deposition, sputtering or a combination thereof to meet the customers' specific needs. A protective overcoat may be applied onto the sealed microcups or the second electrode layer to further improve the optical or physicomechanical properties of the finished panel. The finished display panel is then ready for module assembly.
This new product concept significantly simplifies the manufacturing process and reduces cost. To enable this product concept, an adhesive or sealing layer having a long green time before lamination and fast post curing rate after lamination onto an electrode layer or substrate is highly desirable.